Steel sheets to be stamped (press-formed) and used typically in automobiles and industrial machines require both satisfactory strengths and excellent ductility. High-strength, high-ductility steel sheets have been developed so as to ensure collision safety and weight reduction of automobiles, while satisfying the aforementioned requirements. A TRIP-aided steel sheet is listed as one of them. The TRIP-aided steel sheet includes retained austenite (γR) formed in the structure and effectively utilizes such a property that the γR undergoes induced transformation (strain induced transformation: TRIP) during work deformation to help the steel sheet to have better ductility (see, for example, PTL 1).
The TRIP-aided steel sheet is, however, disadvantageously inferior in workability [particularly in stretch flangeability (bore expandability)] so as to allow easy working into a complicated shape. The stretch flangeability is a property necessary for steel sheets for use typically as undercarriage parts of automobiles. Thus, a strong demand has been made to improve stretch flangeability in a TRIP-aided steel sheet also in order to promote the application of the TIP steel sheet typically to undercarriage parts where the weight reduction effect by the TRIP-aided steel sheet is most expected.
Under these circumstances, the present applicants made various investigations so as to provide a steel sheet which maintains excellent strength-ductility balance by the action of γR and excels also in formability such as stretch flangeability. The investigations were made while focusing attention on effects of warm working to improve the stretch flangeability (see, for example, NPL 1 to 3). As a result, they found that a steel sheet, when being suitably controlled in average hardness of the matrix structure, carbon concentration in γR as a second phase, and volume fraction of γR and being subjected to warm working, can give a high-strength steel sheet having both better stretch flangeability and better elongation. An invention was made based on these findings (hereinafter referred to as “prior invention,” and a high-strength steel sheet according to the prior invention is referred to as a “steel sheet of the prior invention”), and a patent application was already filed on this invention (see PTL 2).
The steel sheet of the prior invention is a high-strength steel sheet containing, on the percent by mass basis:
carbon (C) in a content of from 0.05% to 0.6%,
silicon (Si) and aluminum (Al) in a total content of from 0.5% to 3%,
manganese (Mn) in a content of from 0.5% to 3%,
phosphorus (P) in a content of 0.15% or less (excluding 0%), and
sulfur (S) in a content of 0.02% or less (including 0%),
in which the steel sheet has a matrix structure containing 70 percent by area or more of bainitic ferrite and/or granular bainitic ferrite relative to the total structure, the bainitic ferrite and/or granular bainitic ferrite having an average hardness in terms of Vickers hardness of 240 Hv or more,the steel sheet has a second phase structure containing 5 to 30 percent by area of retained austenite relative to the total structure, and the retained austenite has a carbon concentration (CγR) of 1.0 percent by mass or more, andthe steel sheet may further contain bainite and/or martensite.
PTL 2 mentions that the steel sheet of the prior art has good properties probably because γR itself exhibits maximum plastic stability particularly in a temperature range of from 100° C. to 400° C. (preferably from 150° C. to 250° C.); and that this is achieved by controlling the structure as above and thereby suitably controlling the CγR (carbon concentration in γR) and the hardness of the matrix structure, where CγR significantly affects the TRIP effect due to strain induced transformation of γR, and the hardness of the matrix structure significantly affects the space constraint state of γR (see Paragraph [0023] in PTL 2).
Particularly PTL 2 mentions that, from the viewpoint of exhibiting a TRIP (strain induced transformation working) effect, the steel sheet of the prior invention should essentially have a carbon concentration in γR (CγR) of 1.0 percent by mass or more; and that the larger CγR is, the better (see Paragraph [0030] in PTL 2).
However, after further investigations, the present inventors have found that the TRIP effect is maximally exhibited in warm working (100° C. to 250° C.) where the driving force of the stress-induced transformation upon deformation becomes small by controlling the CγR to a lower range of less than 1.0 percent by mass, which is lower than the specific range (1.0 percent by mass or more) in the prior invention; and that a steel sheet having further better ductility than that of the steel sheet of the prior invention, though slightly sacrificing stretch flangeability, can be obtained by further introducing a specific amount of polygonal ferrite.